Large oblong plastic tanks, such as those for containing water or wastewater have in the past been fabricated by molding two dish-shape halves and then mating the halves at a lengthwise planar joint. This kind of construction provides an advantage over one piece plastic tanks in that half tanks may be nested for shipment and storage, potentially reducing handling and transport costs. It also enables closer control of wall thickness and better definition of interior detail. This kind of construction provides an economic and ease-of-handling advantage over typical concrete septic tanks that are heavy and comprised of halves which do not nest for transport.
In the past, half tanks may be joined to each other by adhesives or mechanical means. Adhesives have the advantage of permanency of joint. Examples of tanks made by such means include those shown in Graham U.S. Pat. No. 4,325,823 and U.S. Patent Publication 2001/0019026 of Berg et al. Mechanical joining means provide the advantage of being independent of substrate material properties, joint preparation, environment, and cure time which often relate to bonding or fusing.
Large plastic tanks, including those suitable for septic tank application, have been previously made as injection molded halves which are clamped together for use. See for example, U.S. Pat. Nos. 7,572,372 and 5,878,907 of Graf. Injection molding produces good dimensional control and fidelity, compared to tanks made by fiberglass resin layup or to polyolefin tanks made in one or more pieces by blow molding or rotational molding as shown in Kruger U.S. Pat. No. 8,070,005.
When a septic tank is an assembly of mechanically mated halves, it is important that the seal of the joint between half tanks be sound, to prevent flow into or out of the tank through the joint region. In contrast to typical concrete septic half tanks which are typically mated at the point of installation in the soil, molded plastic septic tanks of the type described in the Graf patents and the present invention can be assembled at a work place remote from the factory and then transported a relatively short distance to the point of use, where they may be lifted from a truck and placed in a pit. Often an assembled tank is dragged across the surface of the earth to the point of installation. Occasionally, a tank may be dropped from a truck bed or other transporter. Thus, the means for holding the halves together must be sufficient to reasonably endure such kinds of mechanical forces.